Video compression has been a popular subject for academia, industry and international standards bodies alike for more than two decades. Consequently, many compressors/decompressors, or coders/decoders (“codecs”) have been developed providing performance improvements or new functionality over the existing ones. Several video compression standards include MPEG-2, MPEG-4, which has a much wider scope, and H.26L and H.263 that mainly target communications applications.
Some generic codecs supplied by companies such as Microsoft® and Real Networks® enable the coding of generic video/movie content. Currently, the MPEG-4 standard and the H.26L, H.263 standards offer the latest technology in standards-based codecs, while another codec DivX;—) is emerging as an open-source, ad-hoc variation of the MPEG-4 standard. There are a number of video codecs that do not use these or earlier standards and claim significant improvements in performance; however, many such claims are difficult to validate. General purpose codecs do not provide significant improvement in performance. To obtain significant improvements, video codecs need to be highly adapted to the content they expect to code.
The main application of video codecs may be classified in two broad categories based on their interactivity. The first category is interactive bi-directional video. Peer-to-peer communications applications usually involve interactive bi-directional video such as video telephony. In video telephony, the need exists for low delay to insure that a meaningful interaction can be achieved between the two parties and the audio and video (speaker lip movements) are not out of synchronization. Such a bi-directional video communication system requires each terminal both to encode and decode video. Further, low delay real-time encoding and decoding and cost and size issues require similar complexity in the encoders and decoders (the encoder may still be 2-4 times more complex than the decoder), resulting in almost a symmetrical arrangement.
The second category of video codecs relates to video distribution applications, including broadcast and Video-on-Demand (VoD). This second category usually does not involve bi-directional video and, hence, allows the use of high complexity encoders and can tolerate larger delays. The largest application of the second group is entertainment and, in particular, distribution of full-length movies. Compressing movies for transmission over the common broadband access pipes such as cable TV or DSL has obvious and significant applications. An important factor in delivering movies in a commercially plausible way includes maintaining quality at an acceptable level at which viewers are willing to pay.
The challenge is to obtain a very high compression in coding of movies while maintaining an acceptable quality. The video content in movies typically covers a wide range of characteristics: slow scenes, action-packed scenes, low or high detailed scenes, scenes with bright lights or shot at night, scenes with simple camera movements to scenes with complex movements, and special effects. Many of the existing video compression techniques may be adequate for certain types of scenes but inadequate for other scenes. Typically, codecs designed for videotelephony are not as efficient for coding other types of scenes. For example, the International Telecommunications Union (ITU) H.263 standard codec performs well for scenes having little detail and slow action because in video telephony, scenes are usually less complex and motion is usually simple and slow. The H.263 standard optimally applies to videoconferencing and videotelephony for applications ranging from desktop conferencing to video surveillance and computer-based training and education. The H.263 standard aims at video coding for lower bit rates in the range of 20-30 kbps.
Other video coding standards are aimed at higher bitrates or other functionalities, such as MPEG-1 (CDROM video), MPEG-2 (digital TV, DVD and HDTV), MPEG-4 (wireless video, interactive object based video), or still images such as JPEG. As can be appreciated, the various video coding standards, while being efficient for the particular characteristics of a certain type of content such as still pictures or low bit rate transmissions, are not optimal for a broad range of content characteristics. Thus, at present, none of the video compression techniques adequately provides acceptable performance over the wide range of video content.
FIG. 1 illustrates a prior art frame-based video codec and FIG. 2 illustrates a prior art object based video codec. As shown in FIG. 1, a general purpose codec 100 is useful for coding and decoding video content such as movies. Video information may be input to a spatial or temporal downsampling processor 102 to undergo fixed spatial/temporal downsampling first. An encoder 104 encodes video frames (or fields) from the downsampled signal. An example of such an encoder is an MPEG-1 or MPEG-2 video encoder. Encoder 104 generates a compressed bitstream that can be stored or transmitted via a channel. The bitstream is eventually decoded via corresponding decoder 106 that outputs reconstructed frames to a postprocessor 108 that may spatially and/or temporally upsample the frames for display.
FIG. 2 shows a block diagram of a specialized object-based codec 200 for coding and decoding video objects as is known in the art. Video content is input to a scene segmenter 202 that segments the content into video objects. A segment is a temporal fragment of the video. The segmenter 202 also produces a scene description 204 for use by the compositor 240 in reconstructing the scene. Not shown in FIG. 2 is the encoder of the scene description produced by segmenter 202.
The video objects are output from lines 206 to a preprocessor 208 that may spatially and/or temporally downsample the objects to output lines 210. The downsampled signal may be input to an encoder 212 such as a video object encoder using the MPEG-2, MPEG-4 or other standard known to those of skill in the art. The contents of the MPEG-2, MPEG-4, H.26L and H.263 standards are incorporated herein by reference. The encoder 212 encodes each of these video objects separately and generates bitstreams 214 that are multiplexed by a multiplexer 216 that can either be stored or transmitted on a channel 218. The encoder 212 also encodes header information. An external encoder (not shown) encodes scene description information 204 produced by segmenter 202.
The video objects bitstream is eventually demultiplexed using a demultiplexer 220 into individual video object bitstreams 224 and are decoded in video object decoder 226. The resulting decoded video objects 228 may undergo spatial and/or temporal upsampling using a postprocessor 230 and the resulting signals on lines 232 are composed to form a scene at compositor 240 that uses a scene description 204 generated at the encoder 202, coded by external means and decoded and input to the compositor 240.
Some codecs are adaptive in terms of varying the coding scheme according to certain circumstances, but these codecs generally change “modes” rather than address the difficulties explained above. For example, some codecs will switch to a different coding mode if a buffer is full of data. The new mode may involve changing the quantizer to prevent the buffer from again becoming saturated. Further, some codecs may switch modes based on a data block size to more easily accommodate varying sized data blocks. In sum, although current codecs may exhibit some adaptiveness or mode selection, they still fail to address the inefficiencies in encoding and decoding a wide variety of video content using codecs developed for narrow applications.